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The mosquito-borne Zika virus causes birth defects, including a syndrome in which babies are born with unusually small heads, American health authorities confirmed on Wednesday after months of debate and uncertainty.

"Scientists at the Centres for Disease Control and Prevention have concluded, after careful review of existing evidence, that Zika virus is a cause of microcephaly and other severe fetal brain defects," said a statement from the federal agency.

The decision was based in part on a series of studies in Brazil, where thousands of babies were born last year with birth defects, coinciding with a spike in Zika virus infections among the general public.

Tom Frieden, head of the CDC, said: "This study marks a turning point in the Zika outbreak.

"It is now clear that the virus causes microcephaly."

An electron micrograph (TEM) of the Zika virusCredit:
NCEZID

There was no "smoking gun," or single piece of evidence that offered conclusive proof, said the full report, published in the New England Journal of Medicine.

Rather, the decision was made based on "increasing evidence from a number of recently published studies and a careful evaluation using established scientific criteria," said the CDC.

Further studies are being launched to "determine whether children who have microcephaly born to mothers infected by the Zika virus is the tip of the iceberg of what we could see in damaging effects on the brain and other developmental problems," Dr Frieden added.